1. Field of the Invention
The present invention relates to a monolithic ceramic capacitor, and in particular, to an LW-reverse-type monolithic ceramic capacitor including external terminal electrodes each including a resistance component.
2. Description of the Related Art
In a power supply circuit, when a voltage variation in a power supply line is increased by an impedance that is present in the power supply line or a ground, the operation of circuits to be driven becomes unstable, interference between the circuits occurs due to the power supply circuit, or oscillation occurs. Consequently, a decoupling capacitor is usually connected between the power supply line and the ground. The decoupling capacitor decreases the impedance between the power supply line and the ground, thereby suppressing the variation in the power supply voltage and interference between the circuits.
Recently, in communication equipment such as a cell phone and information processing equipment such as a personal computer, as the speed of signals has been increased in order to allow a large amount of information to be processed, the clock frequency of an IC used has also increased. Accordingly, noise that primarily includes harmonic wave components is often generated. Therefore, it has become necessary to provide stronger decoupling in an IC power supply circuit.
In order to increase the decoupling effect, it is effective to use a decoupling capacitor having an excellent impedance-frequency characteristic. An example of such a decoupling capacitor is a monolithic ceramic capacitor. Because of its low equivalent series inductance (ESL), the monolithic ceramic capacitor has an excellent noise-absorbing effect over a wide frequency range as compared to an electrolytic capacitor.
Another function of a decoupling capacitor is to supply electric charges to an IC. A decoupling capacitor is usually disposed in the vicinity of an IC. When a voltage variation occurs in a power supply line, electric charges are rapidly supplied from the decoupling capacitor to the IC, thus preventing a delay of the IC.
When a charge and a discharge occur in a capacitor, a counter-electromotive force represented by a formula dV=L·di/dt is generated in the capacitor. With a large dV, the supply speed of electric charges to the IC is decreased. With an increase in the clock frequency of an IC, the amount of current variation per unit time di/dt tends to increase. Accordingly, in order to decrease the value of dV, it is necessary to decrease the inductance L. For this purpose, it is desirable to further decrease the ESL of a capacitor.
A known example of a low-ESL monolithic ceramic capacitor in which the ESL is further decreased is an LW-reverse-type monolithic ceramic capacitor. In typical monolithic ceramic capacitors, the dimension (dimension W) of each end surface of a capacitor main body in the extending direction of the ceramic layers, the end surface having an external terminal electrode thereon, is less than the dimension (dimension L) of each side surface of the capacitor main body in the extending direction of the ceramic layers, the side surface being adjacent to the end surfaces. In contrast, in LW-reverse-type monolithic ceramic capacitors, the dimension (dimension W) of each end surface in the extending direction of the ceramic layers, the end surface having an external terminal electrode thereon, is greater than the dimension (dimension L) of each side surface in the extending direction of the ceramic layers. In such LW-reverse-type monolithic ceramic capacitors, a current path of a capacitor main body is wide and short, thereby decreasing the ESL.
Another known example of a low-ESL monolithic ceramic capacitor is a multiterminal monolithic ceramic capacitor. In multiterminal monolithic ceramic capacitors, the current path inside a capacitor main body is separated into a plurality of paths, thereby decreasing the ESL.
In low-ESL monolithic ceramic capacitors, the current path is wide and short or is separated as described above. As a result, the equivalent series resistance (ESR) is also decreased.
On the other hand, an increase in the capacitance of monolithic ceramic capacitors has been required. In order to increase the capacitance of a monolithic ceramic capacitor, the number of ceramic layers and the number of laminated internal electrodes may be increased. In this case, the number of current paths is increased, thereby decreasing the ESR.
Accordingly, in response to the requirements to decrease the ESL and increase the capacitance, the ESR of monolithic ceramic capacitors tends to be further decreased.
However, it is known that when the ESR of a capacitor is excessively decreased, a mismatch of impedance occurs in a circuit and a damped oscillation called “ringing” in which the rising of a signal waveform deforms easily occurs. The ringing may cause a malfunction of an IC because of disordered signals.
In addition, when the ESR of a capacitor is excessively decreased, the impedance-frequency characteristic of the capacitor becomes excessively steep near the resonance frequency. More specifically, the valley of the impedance curve becomes excessively deep. Consequently, it may be difficult to absorb noise over a wide frequency range.
In order to prevent ringing or to broaden the impedance-frequency characteristic, a resistance element may be connected in series to a line. In addition, recently, it has been required that a capacitor itself includes a resistance component, and thus, a method of controlling the ESR of such a capacitor using this technique has attracted attention.
For example, Japanese Unexamined Patent Application Publication No. 2004-47983 (document '983) and PCT Publication No. WO 2006/022258 pamphlet (document '258) have disclosed that a resistance component is included in external terminal electrodes that are electrically connected to internal electrodes, thereby controlling the ESR. More specifically, document '983 discloses a thick-film resistance including RuO2. Document '258 discloses that paste including a material having a relatively high specific resistance, such as ITO, is baked on a capacitor main body. However, the techniques described in documents '983 and '258 have problems to be solved as described below.
According to the technique disclosed in document '983, a plating film is formed directly on an underlayer including the resistance component. However, unlike metal particles, necking does not occur in metal oxide particles, such as RuO2 particles, included in the underlayer by baking. Therefore, the density of the resulting film is not significantly high. Consequently, a plating solution or moisture easily intrudes into the film, thus causing a problem of reduced reliability.
In the technique disclosed in document '258, a first layer including a resistance component is completely covered with a second layer composed of a thick film including a metal such as Cu, and a plating film is formed on the second layer. In this configuration, since the first layer is covered with the dense second layer, the reliability of the capacitor is improved as compared to the capacitor disclosed in document '983. However, since the entire thickness of each of the external terminal electrodes is increased by forming the first layer and the second layer, the dimensions of the monolithic ceramic capacitor in the in-plane directions and the height direction increase. Accordingly, it is difficult to reduce the size of the monolithic ceramic capacitor. This problem tends to be particularly troublesome in LW-reverse-type monolithic ceramic capacitors, which have a large area of external terminal electrodes.
An external terminal electrode is formed on each end surface of a capacitor main body. In order to achieve satisfactory mountability, the external terminal electrode typically has a wrap-around portion which is formed so as to extend from an end surface to principal surfaces and side surfaces. As described in document '258, when the first layer is completely covered with the second layer, the second layer is affected by a variation in the thickness of the first layer. Therefore, it is difficult to stabilize the dimensions of the wrap-around portion. If the dimensions of the wrap-around portion vary, the mountability may be adversely affected.